1. Field of the Invention
This invention relates to vehicle navigation and more particularly to display of data to a user indicating road sign information for route guidance. This disclosure relates to that of copending U.S. patent application Ser. No. 08/689,944, filed Aug. 16, 1996, entitled "Sign Text Display Method and Apparatus for Vehicle Navigation System", incorporated herein by reference in its entirety.
2. Description of Related Art
Vehicle navigation systems are well known; see e.g. Yamada et al., U.S. Pat. No. 4,926,336; Link et al., U.S. Pat. No. 5,270,937; and Ichikawa, U.S. Pat. No. 5,410,485. Such navigation systems typically include two major functions. One function is route guidance which determines the optimum route to be followed from a starting point to a destination, typically along a network of roads. The other function is displaying to the user of the system (typically the driver of the vehicle) both the current location and the route to be followed. This display is usually in the form of a map displayed on a small display screen installed in the vehicle. Such systems are well known.
As is well known, typically in such systems a database is provided of the road network. This database lists all of the roads for instance in one state or in some other fairly large geographical area. This representation of the road network is sometimes referred to as a digitized map database. It includes for each road the road name, and divides each road into one or more road segments, where a road segment extends from one road node to another. A road node is typically either an intersection with another road or an intersection of a road with some arbitrary boundary, e.g. a state boundary and has an associated latitude and longitude.
In addition to providing the actual road information for use by the system, a typical digitized map database also includes road sign information. That is, when the database is compiled, i.e. by a survey of the roads, the surveyor also enters into the database the textual contents of at least some of the actual road signs encountered along each road segment. This road sign information is used in the prior art for route guidance, especially on roads such as freeways or equivalents (multi-lane limited access roads). When the system provides route guidance such as from a freeway exiting onto a surface street, road sign information from the database, referred to in the art as "sign text" is displayed on the display screen to the user. This sign text allows the driver to recognize the actual road sign information at a bifurcation in the road, i.e. a node, and more easily decide which road to follow. Thus this display of sign text enhances the route guidance function.
Hence the sign text data is usually an edited version of the actual text of the road sign and is often not the identical words nor arranged identically. Typically the sign text data is edited to make it meaningful for use in the database. This editing is a part of the compilation of the database. It is to be understood that typically the database is produced and sold separately (e.g as a CD ROM) from the actual navigation system since such a navigation system must be useful anywhere.
In some cases the route guidance instructions are provided not only by a visual display but also by announcement, i.e. by sound synthesis. The present invention is applicable both to systems using a display and systems having adequate sound synthesis capability. Hence the term "display" is used here to refer not only to a visual display, but also to an audible announcement by the system.
This invention relates to copending U.S. patent application Ser. No. 08/689,944, filed Aug. 16, 1996, inventor Roderick Poonsaengsathit and entitled "Sign Text Display Method and Apparatus for Vehicle Navigation System". That disclosure is similarly directed to guidance of a driver on road segments where the road segments connect to a freeway.
As shown in present FIG. 1, the driver intends to follow a guided route from a freeway A to a street A via freeway connector road segments 1, 2, 3. There are several decision points on these freeway connectors (freeway exit ramps), and route sign data for each decision point is associated with the corresponding freeway connector road segment. Assuming that the actual road sign at the first decision point DPT1 is Fwy B and Fwy C, the freeway connector road segment 1 between the first decision point DPT1 and the second decision point DPT2 has Fwy B/Fwy C as the associated sign text data. Usually, a sign text data includes branch segment data for the subsequent decision point. (Note that the display images shown associated with each guidance point in FIG. 1 are discussed later, and do not form a part of this explanation of the problem.)
FIG. 1 shows these actual route signs: Fwy B and Fwy C at decision point DPT1, Fwy B, Fwy C and A street at decision point DPT2, and Fwy B and A street at decision point DPT3. The following associated sign text data are stored in the road map data base: Fwy B/Fwy C for freeway connector road segment 1, Fwy B/A street for freeway connector road segment 2, A street for freeway connector road segment 3, and Fwy B for freeway connector road segment 4. (Note that the sign text data may not be identical to the actual road signs; this is conventional.)
In the conventional system, the sign text data Fwy B/Fwy C for freeway connector road segment 1 is displayed by the system at the first guidance point GPT1, which is located at a certain distance before DPT1; the sign text data Fwy B/A street for freeway connector road segment 2 is displayed at guidance point GPT2 after passing DPT1 and before DPT2; the sign text data A street for freeway connector road segment 3 is displayed at guidance point GPT 3 after passing DPT2 and before reaching DPT3. The guidance points do not necessarily correspond to the location of the actual corresponding road signs, but instead are located at some distance before the associated decision point to allow the driver sufficient time to read the sign text data from the display and respond, in terms of turning in the correct direction at the subsequent decision point.